From: Piotr Krysiuk piotras@gmail.com
commit 10d2bb2e6b1d8c4576c56a748f697dbeb8388899 upstream.
retrieve_ptr_limit() computes the ptr_limit for registers with stack and map_value type. ptr_limit is the size of the memory area that is still valid / in-bounds from the point of the current position and direction of the operation (add / sub). This size will later be used for masking the operation such that attempting out-of-bounds access in the speculative domain is redirected to remain within the bounds of the current map value.
When masking to the right the size is correct, however, when masking to the left, the size is off-by-one which would lead to an incorrect mask and thus incorrect arithmetic operation in the non-speculative domain. Piotr found that if the resulting alu_limit value is zero, then the BPF_MOV32_IMM() from the fixup_bpf_calls() rewrite will end up loading 0xffffffff into AX instead of sign-extending to the full 64 bit range, and as a result, this allows abuse for executing speculatively out-of- bounds loads against 4GB window of address space and thus extracting the contents of kernel memory via side-channel.
Fixes: 979d63d50c0c ("bpf: prevent out of bounds speculation on pointer arithmetic") Signed-off-by: Piotr Krysiuk piotras@gmail.com Co-developed-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/bpf/verifier.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2740,13 +2740,13 @@ static int retrieve_ptr_limit(const stru case PTR_TO_STACK: off = ptr_reg->off + ptr_reg->var_off.value; if (mask_to_left) - *ptr_limit = MAX_BPF_STACK + off; + *ptr_limit = MAX_BPF_STACK + off + 1; else *ptr_limit = -off; return 0; case PTR_TO_MAP_VALUE: if (mask_to_left) { - *ptr_limit = ptr_reg->umax_value + ptr_reg->off; + *ptr_limit = ptr_reg->umax_value + ptr_reg->off + 1; } else { off = ptr_reg->smin_value + ptr_reg->off; *ptr_limit = ptr_reg->map_ptr->value_size - off;